Wind energy can be converted to electrical energy by large central wind turbine farms located on hill tops, off shore and at other windy locations. But these wind turbines are very large, high and expensive, are unsightly, and can harm the natural fauna with the long spinning blades. The efficiency of these systems is limited and even in areas of regular wind such as along the shore or on mountain tops, the energy produced is orders of magnitude less than conventional steam electric stations. Suitable locations for these wind farms are also limited making wind energy conversion only a very small part of the green energy technology portfolio.
An effective alternative to wind farms and large wind turbines is the individual wind mill located in each residence. Such a distributed system has many advantages including relatively low capital investment through the individual homeowner, the ability to utilize the energy on site, eliminating much of the transmission loss associated with central stations, and the fact that the wind is usually blowing somewhere making the overall effectiveness of a distributed wind energy system better than a large central wind energy farm. Since residential buildings are ubiquitous and widely distributed, both in the cities and in suburban and rural areas, it is possible, with the right economic incentives to have tens of millions of wind energy conversion systems on homes throughout the country.
Current residential wind energy systems, whether the conventional horizontal or the vertical axis configuration, suffer from several severe limitations leading to low adoption rates:                Residential buildings are not structurally built to handle the high forces of a windmill on the roof. Consequently, there is a need for building a high tower on the residence property to mount the blade system and this is both expensive and unsightly. Even for much smaller windmills that could be roof mounted, the noise of the blades passing by the support structure is typically not acceptable for a homeowner.        Windmill theoretical maximum efficiency is given by the Betz limit wherein no more than 59% of the energy in the wind may be converted to useful work. This is because for a windmill propeller system, the wind velocity would have to approach zero to extract all of the kinetic energy, and this is impossible since the wind must exit the blade system.These issues combine to make residential windmills impractical for the typical homeowner, leading to very low adoption rates. What is needed is a system that can capture a large cross sectional area of the wind on a roof top and to extract that energy with a small highly efficient turbine, eliminating the theoretical Betz limit and greatly reducing the noise without the need for a large tower or capital investment in a high structure, since the roof is typically high above ground level and in the wind already. For many thousands of years wind has been captured to do useful work using sail technology. Sails can present a large cross sectional area to catch the wind such as in a sailing vessel. In ancient Rome, the shading of the Coliseum was accomplished by sailors with large awning like sail structures borrowed from the sail technology. The deployment and management of the sails can be accomplished with lines as sheets, guys and winches as well as poles and booms to secure the sail position. This technology allows reefing of the sail in high wind conditions and greater area during lighter airs, as opposed to the typical bladed windmill with constant cross sectional area. Sail technology also has the advantage of allowing the most efficient presentation to the wind, and the sail can be completely furled during snowstorms or inclement weather. A roof top sail also has the advantage of presenting a large area to the sun so that solar cells could be mounted on top of it, then furled during snowstorms to keep the solar cells clear of snow. And a retractable sail would look reasonable if deployed from the roof peak ridge and could be made of colorful fabric such as spinnaker sails on yachts.        